The present invention relates to an electric double layer device, and, more particularly to an electric double layer device configured such that a lower terminal is directly withdrawn from a lower collecting plate in the same manner as the manner in which an upper terminal is directly withdrawn from an upper collecting plate, thereby improving productivity and ease of assembly, increasing connection force, and improving discharge efficiency during the discharge of high current while reducing internal equivalent resistance.
In general, an electric double layer device is a device such as a battery, a capacitor, or an electrolytic condenser, which stores electrical energy. The electric double layer device electrically charged and discharged using electrodes that are electrically conductive. Electric double layer devices are used in cellular phones, GPS receivers, MP3 players, and backup memories. In addition, electric double layer devices are used in wind energy systems, solar energy systems, and motors of electric vehicles and hybrid electric vehicles.
An electric double layer is a layer configured such that positive charges are successively positioned on one surface of a thin film layer of an object and negative charges are successively positioned on the other surface of the thin film layer or such that both surfaces of the thin film layer have the same surface density. The electric double layer is typically a double layer that includes electric dipoles. In general, charges are rearranged at the interface between different materials, whereby an electric double layer is formed.
Positive ions or negative ions in a liquid-state aqueous electrolytic solution are selectively adsorbed at the interface between a solid-state electrode and a liquid-state aqueous electrolytic solution, solid surface molecules are dissociated, or the positive ions or the negative ions are adsorbed on the dipole interface, whereby an electric double layer is formed. This layer is referred to as a Helmholtz layer.
Such an electric double layer is closely related to various electrochemical surface phenomena, such as an electrode reaction, an electrokinetic phenomenon (an electrophoretic phenomenon), and the stability of colloids.
One example of such an electric double layer device may be a capacitor.
In the electric double layer capacitor, an electrostatic layer is formed at the interface between an activated carbon electrode and an organic electrolyte, and an electric double layer state is used as the function of a dielectric to accumulate electricity in the same manner as in a battery.
In particular, charges that accumulate in an electric double layer, formed between a solid electrode and a solid-state or liquid-state electrolyte, are used.
The capacitor has lower energy density than the battery. However, the capacitor is superior to the battery in terms of power density, that is, instantaneous high output. In addition, the capacitor is usable hundreds of thousands of times. That is, the lifespan of the capacitor is semi-permanent. For these reasons, capacitors are used in various fields.
The electric double layer capacitor is operated according to the following principle. When direct current voltage is applied to a pair of solid electrodes in a state in which the solid electrodes are placed in an electrolyte ion solution, negative ions are electrostatically drawn to an electrode polarized as a positive electrode, and positive ions are electrostatically drawn to an electrode polarized as a negative electrode. As a result, an electric double layer is formed at the interface between each electrode and the electrolyte. In particular, activated carbon has a plurality of pores. For this reason, the electric double layer is easily formed. The capacitance of the stored charge may be calculated using Equation 1 below.
                    C        =                                                            ɛ                0                            ⁢              ɛ                                      4              ⁢              πσ                                ⁢                      ∫            ds                                              [                  Equation          ⁢                                          ⁢          1                ]            
Where ε0 indicates the permittivity of air, ε indicates the permittivity of an electrolyte, σ indicates the radius of electrolytic ions, and S indicates the specific surface area of an electrode.
The factors that determine the capacity of an electric double layer capacitor are as follows. As can be seen from Equation 1 above, the larger the specific surface area of the electrode, the larger the permittivity of the electrolyte, and the smaller the radius of the ions during the formation of the double layer, the greater the capacity that can be obtained. In addition, capacitance is determined by the internal resistance of the electrode, the relationship between the distribution of pores in the electrode and the electrolytic ions, internal voltage, etc.
The electric double layer capacitor includes electrodes, a separator, an electrolyte, current collectors, and a case.
The selection of materials for the electrodes is most important when configuring the capacitor. However, the capacitance of the capacitor is changed by various other components of the capacitor.
The materials for the electrodes must have high electrical conductivity and a large specific surface area. In addition, the materials for the electrodes must be electrochemically stable.
Another example of such an electric double layer device may be a battery.
The battery is a device that converts chemical energy, stored in a chemical material (i.e. an active material) contained therein, into electrical energy through an electrochemical oxidation-reduction reaction (redox reaction).
The battery is constituted by an assembly of two or more electrochemical cells. Alternatively, the battery may be constituted by a single cell. The battery is configured such that electrons flow to the outside along a conducting wire due to an electrochemical reaction, rather than a chemical reaction. The electrons flowing along the conducting wire becomes the source of electrical energy, thereby being electrically useful.
More specifically, the battery has a positive electrode (cathode) active material and a negative electrode (anode) active material coated on a current collector. The positive electrode and the negative electrode are separated from each other by a separator. In addition, the positive electrode and the negative electrode are contained in an electrolyte, which enables the transfer of ions between the two electrodes.
In order to operate an electric lamp, an apparatus, an instrument, etc., the electrode materials and electrolyte must be selected appropriately and arranged so as to have a specific structure such that sufficient voltage and current are generated between the two electrodes of the battery.
For example, a positive electrode, the positive electrode active material of which is reduced by electrons received from an external conducting wire, a negative electrode, the negative electrode active material of which is oxidized so as to emit electrons to the conducting wire, an electrolyte, which enables a material to move such that the reduction reaction of the positive electrode and the oxidation reaction of the negative electrode are chemically harmonious, and a separator, which prevents physical contact between the positive electrode and the negative electrode, must be arranged so as to convert chemical energy into electrical energy based on interactions therebetween.
The negative electrode of the battery, arranged as described above, basically emits electrons while being oxidized, and the positive electrode receives electrons while being reduced (together with positive ions). When the battery is operated in a state of being connected to an external load, therefore, the two electrodes are electrochemically changed to thus perform electrical work.
At this time, the electrons, which are generated by the oxidation reaction of the negative electrode, move to the positive electrode via the external load, and undergo a reduction reaction with the positive electrode active material. Consequently, the flow of charges is completed as the result of movement of anions (negative ions) and cations (positive ions) toward the negative electrode and the positive electrode in the electrolyte.
In this way, the reaction is performed in the electrolyte such that charges continuously flow in the external conducting wire, and the electrical operation is performed using the charges.
Based on the kind of an electrolytic solution, the battery may be classified as a liquid electrolyte battery or a polymer electrolyte battery. In general, the liquid electrolyte battery is referred to as a lithium ion battery, and the polymer electrolyte battery is referred to as a lithium polymer battery.
FIG. 1 is a schematic view showing the structure of a general electric double layer device, FIG. 2 is a schematic view illustrating a principle whereby an electric double layer capacitor, applied to a general electric double layer device, is charged, and FIG. 3 is a circuit diagram illustrating a principle whereby the electric double layer capacitor applied to the general electric double layer device is charged and discharged.
As shown in FIG. 1, a general electric double layer device 100 includes electrodes 10, an electrolytic solution 20, current collectors 30, a separator 40, a first lead terminal 61, and a second lead terminal 62.
On the assumption that the electric double layer device 100 is a battery, the chemical energy of a chemical material (i.e. an active material) therein is converted into electrical energy through an electrochemical oxidation-reduction reaction (redox reaction), and the electrodes 10, which are put on the current collectors 30, have a positive electrode and a negative electrode as the active material.
Describing the characteristics of the electric double layer device 100 in more detail based on the assumption that the electric double layer device 100 is a capacitor, on the other hand, energy is stored using the distribution of positive and negative charges which are arranged within a short distance from each other at the interface between the two different electrodes 10, the capacitance, in farads, is high, and the change, and deterioration in performance upon repeated charge and discharge cycles thereof are very low.
The electrodes 10 are made of activated carbon, which has a large specific surface area, and store charges generated at the electric double layer, which is disposed at the interface with the electrolytic solution 20. Of the electrical characteristics of the electrode 10, capacitance and internal resistance are the most important criteria in evaluating the performance thereof. Consequently, the electrodes 10 must exhibit low specific resistance and have a porous structure. In the porous structure, the size and distribution of pores must be simple and biased within a predetermined range. The material characteristics of the electrodes 10 most strongly affect the inherent charge and discharge characteristics of the electric double layer capacitor.
In recent years, therefore, an activated carbon-based material, which has a large specific surface area and is inexpensive, has been mainly used as the material for the electrodes 10, and research into the use of metal oxides and conductive polymers in order to increase energy density has been increasingly conducted.
Meanwhile, an organic solvent, quaternary ammonium salt (organic), or sulfuric acid solution (aqueous solution) is used as the electrolytic solution 20. For the organic solvent electrolytic solution, polycarbonate (PC) and ethyl methyl carbonate (EMC) or PC and dimethoxyethane (DME) may be mixed at a predetermined ratio in order to improve electrical conductivity.
An electric double layer capacitor 100 using an organic electrolytic solution has a capacitance per unit area of 4 to 6 μF/cm2. The electrical conductivity of the organic electrolytic solution is higher than that of the aqueous electrolytic solution. Consequently, the electric double layer capacitor 100 using the aqueous electrolytic solution has a capacitance per unit area of 5 to 10 μF/cm2, which is higher than that of the electric double layer capacitor 100 using the organic electrolyte. However, the electric double layer capacitor 100 using the aqueous electrolytic solution has problems in that the potential window is narrow and decomposition occurs.
Nonwoven fabric, porous polyethylene (PE), or polypropylene (PP) film is used as the separator 40.
The principle whereby the electric double layer capacitor is charged is as follows. As shown in FIG. 1, the two electrodes 10 are placed opposite the electrolytic solution 20 in a state in which the separator 40 is located therebetween. In a state in which electrical energy is not supplied from the outside, as shown in FIG. 2, which is a schematic view illustrating the principle whereby the electric double layer capacitor is charged, the electric double layer capacitor is in a bulk state, in which charges are non-uniformly distributed. As a result, the potential difference between the electrodes 10 becomes 0. When electrical energy is supplied from the outside, as shown in FIG. 3, which is a circuit diagram illustrating the principle whereby the electric double layer capacitor is charged and discharged, charges are uniformly distributed throughout the electric double layer capacitor. As a result, as shown in FIG. 2, an energy having voltage corresponding to a potential difference of 2Φ1 is charged between the two electrodes 10.
At this time, even when the supply of electrical energy is interrupted, the electric double layer, which has already been formed, is not extinguished, and therefore the charged electrical energy is retained.
Related Art Document 1 (10-2008-0044054: Module Type Electric Double Layer Capacitor and Method of Manufacturing the Same)
FIG. 4 is a view showing a process of manufacturing an electric double layer capacitor according to Related Art Document 1, FIG. 5 is a view illustrating a method of manufacturing an integrated electric double layer capacitor according to Related Art Document 1, and FIG. 6 is a view illustrating a process of manufacturing an electrode device that constitutes the electric double layer capacitor according to Related Art Document 1.
In general, a secondary battery that can be charged and discharged, for example, an energy storage apparatus, such as an electrolytic condenser or an electrochemical double layer capacitor (EDLC), is configured to have a wound type structure, e.g. a jelly-roll type structure.
As shown in FIG. 4, a wound type energy storage apparatus, such as a wound type electrochemical double layer capacitor, generally includes a cylindrical case 20 made of aluminum (Al) and a wound device 10 mounted in the case 20.
The wound device 10 is formed by winding a strip-shaped electrode stack, that is, positive and negative electrode devices with an electrolyte interposed between the positive and negative electrode devices, in a cylindrical shape and taping the outside of the strip shaped electrode stack in order to prevent the strip-shaped electrode stack from being unwound.
The wound device 10 formed as described above is impregnated with an electrolytic solution, and is mounted in the cylindrical case 20. A terminal plate 30 is installed above the wound device 10, and lug- or screw-type external terminals 40 are fastened to the terminal plate 30.
In addition, a neck 21, which prevents the terminal plate 30 from being pushed downward, is formed in the upper part of the case 20 in a depressed state. The wound device 10 is mounted in the case 20 after the neck 21 is formed in the case 20. The wound device 10 is electrically connected to the external terminals 40 via terminals 120. Subsequently, an upper end 22 of the case 20 is curled. As a result, the terminal plate 30 is fixed in the case 20, and the assembly process is completed.
Referring to the upper part of FIG. 6, an electrode device 100 includes an electrode current collection sheet 111 made of general aluminum foil and an electrode active material 112 applied to the current collection sheet 111.
The electrode active material 112 is formed by applying conductive paste including mostly activated carbon.
The terminal 120 is coupled to the electrode device 100. Specifically, a portion of the electrode device 100 to which the terminal 120 will be coupled is scratched to remove the electrode active material 112 therefrom, the scratched portion of the electrode device 100 is drilled, and the terminal 120 is coupled to the drilled portion of the electrode device 100 by riveting.
The applicant of the present application has improved the electric double layer device having the above-mentioned characteristics, and proposes the improved electric double layer device as the present invention.
Related Art Document 2 (10-2013-0065485: Electric Double Layer Device and Wound Unit for the Same)
FIG. 7 is an exploded perspective view showing an electric double layer device according to Related Art Document 2, FIG. 8 is a sectional view showing the electric double layer device according to Related Art Document 2, FIG. 9A is a plan view showing a wound unit for the electric double layer device according to Related Art Document 2, and FIG. 9B is a half-sectional view showing the wound unit for the electric double layer device according to Related Art Document 2.
As shown in FIGS. 7 to 9B, the electric double layer device according to Related Art Document 2 includes a wound unit 10, which includes a first current collector 11 and a second current collector 12, which are wound while being separated from each other by a separator 10a, and a case 20 having an upper opening 21, through which the wound unit 10 is received, and a lower closure 22.
More specifically, as shown in FIGS. 7 to 9B, the electric double layer device according to Related Art Document 2 further includes a lower connection plate connected to the first current collector 11 of the wound unit 10, a lower insulating plate 32 placed on the lower closure 22 while receiving the lower connection plate 31, a connection core 40 connected to the lower connection plate 31 while being exposed upward in a state in which the connection core 40 is upright along the center of the wound unit 10, one terminal 50a extending upward from an upper collecting plate 50 connected to the second current collector 12 of the wound unit 10, the upper collecting plate 50 having therein a center hole 51, through which the connection core 40 extends, an upper insulating plate 60 fitted on the upper collecting plate 50 excluding the one terminal 50a, the upper insulating plate 60 having therein a through hole 61, through which the connection core 40 extends, and the other terminal 70a connected to the upper end of the connection core 40, the other terminal 70a extending upward while being spaced apart from the one terminal 50a. 
The connection core 40 is perpendicularly connected to the lower connection plate 31, which is connected to the first current collector 11, and then the other terminal 70a is connected to the connection core 40 and is withdrawn. The upper collecting plate 50 is connected to the second current collector 12, and then the one terminal 50a is withdrawn. The one terminal 50a and the other terminal 70a are withdrawn upward.
Furthermore, the wound unit 10 for the electric double layer device according to Related Art Document 2 includes a first current collector 11 and a second current collector 12, which are wound while being separated from each other by a separator 10a, a lower connection plate 31 connected to the first current collector 11, a connection core 40 connected to the lower connection plate 31 while being exposed upward in a state in which the connection core 40 is upright along the center of the first current collector 11 and the second current collector 12, which are wound while being separated from each other by the separator 10a, and one terminal 50a extending upward from an upper collecting plate 50 connected to the second current collector 12 of the wound unit 10, the upper collecting plate 50 having therein a center hole 51, through which the connection core 40 extends.
The one terminal 50a may extend upward from the upper collecting plate 50 and may then be bent. The other terminal 70a may be connected to the upper end of the connection core 40 such that the other terminal 70a is spaced apart from the one terminal 50a. In addition, the other terminal 70a may extend upward and may then be bent.
The lower connection plate 31 is connected to the first current collector 11 of the wound unit 10 in a state in which the connection core 40 is upright along the center of the first current collector 11, for example, such that a negative electrode of the first current collector 11 is withdrawn upward through the connection core 40.
The one terminal 50a is connected to the second current collector 12 of the wound unit 10 in a state in which the one terminal 50a extends upward from the upper collecting plate 50, which has therein the center hole 51 through which the connection core 40 extends, for example, such that a positive electrode of the second current collector 12 is withdrawn upward. The other terminal 70a is connected to the upper end of the connection core 40 while extending upward such that the other terminal 70a is spaced apart from the one terminal 50a. 
At this time, the upper insulating plate 60 is fitted on the upper collecting plate 50 excluding the one terminal 50a such that the one terminal 50a is exposed outward.
The electric double layer device according to Related Art Document 2 may further include an upper nonconductive plate 80 fitted in the case 20 to cover the upper opening 21, the upper nonconductive plate 80 having a first connection terminal 81 and a second connection terminal 82 respectively connected to the other terminal 70a and the one terminal 50a. 
In the electric double layer device according to Related Art Document 2, however, the connection between the other terminal 70a and the connection core 40 is frequently poor since the other terminal 70a is connected to the upper end of the connection core 40 while extending upward. In particular, a plurality of components, such as the connection core 40 and the other terminal 70a, is needed, which make it difficult to perform assembly and connection. As a result, productivity and ease of assembly are considerably reduced. Furthermore, the connection core 40 increases the weight of the electric double layer device.